IV 



THE SEX CHROMOSOMES 



roi 



general use, X standing for the large sex chromosome and Y for the 

 small one, the chromosome formulae of the two species is as follows 

 {Lygaeus having like Proienor six pairs of ordinary chromosomes or 



" Autosomes ") : 



Haploid. 

 eggs all 6 + X. 

 spermatozoa 6 + X or 6. 

 eggs all 6 + X. 

 spermatozoa 6 + X or 6 + Y. 



The cases are seen to be exactly parallel. There are two classes 



X 



Diploid. 

 Protenor ? 12+ XX 



S I2 + X 



Lygaeus ? 12 + XX 



6 12 + XY 





B 



r 



X Y 



%• f I • 



ji 



H 



Y X 



K 



Fig. 46 



The chromosomes of Lygaeus turcicus. (After Wilson, J.E.Z., 1905 and 1912.) A, spermatogonia! 

 telophase ; B, later telophase ; C, emergence of the massive chromatic bodies in the primary spermatocyte ; 

 D, leptotene stage ; E, " confused stage " ; F, evolution of the bivaleats ; G, metaphase I.; H, I, daughter 

 chromosome groups of anaphase I. ; J, metaphase II., X and Y, now paired to form a bivalent ; K, L, daughter 

 chromosome groups from anaphase II. 



of spermatozoa in Lygaeus — one, the 6 +X form, will on fertihzing an egg 

 produce a female, the other (6+Y form) will produce a male. The 

 process of meiosis in the male Lygaeus is illustrated in Fig. 46. This 

 figure demonstrates the important fact that the sex chromosomes of the 

 male often (though not in all cases; see below) remain compact throughout 

 the whole meiotic prophase, including the leptotene and zygotene stages. 

 It will be noted in Fig. 46, G, that X and Y are not paired to form 



